The present invention relates to a damping force control type hydraulic shock absorber attached to a suspension system of a vehicle, for example, an automobile.
Hydraulic shock absorbers attached to suspension systems of automobiles or other vehicles include damping force control type hydraulic shock absorbers which are designed so that the level of damping force can be appropriately controlled in accordance with the road surface conditions, vehicle running conditions, etc. with a view to improving the ride quality and the steering stability.
In general, this type of hydraulic shock absorber includes a cylinder having a hydraulic fluid sealed therein. A piston, which has a piston rod connected thereto to constitute a piston assembly, is slidably fitted in the cylinder to divide the inside of the cylinder into two chambers. The piston assembly is provided with a main hydraulic fluid passage and a bypass passage, which provide communication between the two chambers in the cylinder. The main hydraulic fluid passage is provided with a damping force generating mechanism including an orifice and a disk valve. The bypass passage is provided with a damping force control valve for controlling the flow path area of the bypass passage.
When the bypass passage is opened through the damping force control valve, the flow resistance to the hydraulic fluid flowing between the two chambers in the cylinder is reduced, thereby reducing damping force. When the bypass passage is closed, the flow resistance between the two chambers is increased, thereby increasing damping force. Thus, damping force characteristics can be appropriately controlled by opening and closing the damping force control valve.
In the above-described damping force control type hydraulic shock absorber, in which damping force is controlled by changing the flow path area of the bypass passage, the damping force characteristics can be changed to a considerable extent in the low piston speed region because damping force depends on the restriction by the orifice in the hydraulic fluid passage. However, the damping force characteristics cannot greatly be changed in the intermediate and high piston speed regions because in these regions damping force depends on the degree of opening of the damping force generating mechanism (disk valve, etc.) in the main hydraulic fluid passage.
To solve the above-described problem, Japanese Patent Application Unexamined Publication (KOKAI) No. 62-220728, for example, discloses a damping force control type hydraulic shock absorber in which a pressure chamber (pilot chamber) is formed at the back of a disk valve serving as a damping force generating mechanism in a main hydraulic fluid passage common to the extension and contraction sides, and the pressure chamber is communicated with a cylinder chamber on the upstream side of the disk valve through a fixed orifice and also communicated with a cylinder chamber on the downstream side of the disk valve through a variable orifice (flow control valve).
According to the above damping force control type hydraulic shock absorber, the flow path area of the passage between the two chambers in the cylinder is controlled by opening and closing the variable orifice. Moreover, the valve opening initial pressure of the disk valve can be varied by changing the pressure in the pressure chamber by the pressure loss in the variable orifice. Thus, it is possible to control orifice characteristics (in which damping force is approximately proportional to the square of the piston speed) and valve characteristics (in which damping force is approximately proportional to the piston speed), and hence possible to widen the control range for damping force characteristics.
However, the above-described conventional damping force control type hydraulic shock absorber suffers from the following problems.
In the damping force control type hydraulic shock absorber disclosed in Japanese Patent Application Unexamined Publication (KOKAI) No. 62-220728, damping force is controlled by the flow rate control through the variable orifice. Therefore, damping force actually generated changes according to the piston speed. For this reason, when there is an abrupt input due to thrusting-up force applied to the vehicle from the road surface, for example, damping force increases sharply as the piston speed rises, and shock is transmitted to the vehicle body. This may cause the ride quality to be degraded. Moreover, the flow resistance of the variable orifice varies to a considerable extent according to the viscosity of the hydraulic fluid. Therefore, the effect of temperature changes on the damping force characteristics is unfavorably large. Accordingly, stable damping force characteristics cannot be obtained with the variable orifice.
Meanwhile, there is a semi-active suspension control system in which a damping force control type hydraulic shock absorber is combined with acceleration sensors, a controller, an actuator, etc. to automatically switch over damping force characteristics in real time in accordance with accelerations acting on the vehicle (vertical acceleration, transverse acceleration, longitudinal acceleration, etc.), vehicle running conditions, road surface conditions and so forth, thereby improving the ride quality and the steering stability. It is known that in the semi-active suspension control system, necessary damping force can be obtained rapidly by enabling a combination of different damping force characteristics of the hydraulic shock absorber to be set for the extension and contraction sides, which are different in magnitude of damping force (e.g. a combination of "hard" damping force characteristics for the extension side and "soft" damping force characteristics for the compression side, or vice versa), and thus it is possible to improve the ride quality and the steering stability efficiently and to lighten the load on the controller and the actuator.